The present invention relates to pumps, and more particularly to pump apparatus and methods for pumping molten metal.
The use of pumps to pump molten metal such as aluminum or zinc is known in the art. There are three basic types of molten metal pumps described in detail in prior U.S. Pat. No. 5,203,681. Generally, molten metal pumps comprise centrifugal pumps modified to provide processing of the molten metal. To that end, circulation pumps are used to equalize temperature and improve homogeneity of mixture in a molten metal bath, transfer pumps are used to convey or transfer molten metal between locations and gas-injection pumps are used to circulate and inject gas into a molten metal to modify its composition as by removing dissolved gases or dissolved contaminant metals therefrom.
The pumps typically include a base or casing having a pumping chamber and an impeller received within the chamber. The base includes inlet and outlet passages for intake and discharge of the molten metal being pumped. The pump may be a volute pump wherein the pumping chamber has a volute shape comprising a spiral configuration of circumferentially increasing cross sectional area approaching the pump outlet passage. It is also possible to provide the pump with a pumping chamber having a generally circular shape.
The pump base together with the impeller are submerged in the molten metal and connected via a plurality of support posts to a drive arrangement positioned above the level of the molten metal. The impeller is supported for rotation within the pumping chamber by a rotatable shaft coupled to the drive arrangement. In typical installations, the drive shaft may be of various lengths, e.g. one to four feet in length or longer, in order to provide adequate clearance above the molten metal level.
The portions of the pump submerged in the molten metal are directly contacted and exposed to the harsh conditions thereof, and they are formed of refractory materials such as graphite, silicon carbide, alumina, zirconia or hexaloy. Typical aluminum processing temperatures are in the order of 1200 to 1400xc2x0 F. The aluminum is corrosive at these temperatures and abrasive dross as well as other particulate or solid contaminants are present in the molten metal.
In such aluminum processing, the submerged pump components are typically made of a refractory material such as graphite to inhibit and/or retard damage due to the environment. However, cavitation and turbulence damage is not sufficiently retarded by material selection alone. That is, the violent agitation of the molten metal by the impeller rotation has been found to cause excessive pitting, abrasion and/or spalling of the graphite at the impeller surfaces. In molten metal pumps where the level of the molten metal is reduced during operation, the intimate contact of air and aluminum and/or the oxidizing of the aluminum give rise to a cavitation type defect wherein localized increased concentrations of aluminum oxide and/or turbulent flowing metal worsen the damage to the graphite pump components, especially the impeller. Accordingly, the cavitation and turbulence adjacent the impeller is believed to exacerbate the harshness of the environment and increase the resulting damage. This has been found to be particularly true in respect to the surfaces at the radially interior, low pressure region of the impeller.
A typical impeller includes at least two axially extending vanes and a radially extending member which forms a base when located below the vanes. If the impeller includes a base, the adjacent vanes and base form a pocket which may entrap molten metal when the pump is removed from operation. In such cases, removal of the pump from the molten metal may create a safety hazard due to trapped molten metal. That is, the trapped molten metal may remain molten and subsequently contact a worker withdrawing and/or servicing the pump.
The necessary spacing between the driver and impeller results in the use of an elongate drive shaft fixed to the impeller. This requires a relatively high degree of balance during operation, and accordingly, a bearing support between the impeller/shaft assembly and the housing that is characterized by a high degree of concentricity. Poor concentricity has resulted in sufficient operating vibration to damage prior art pumps. Typically, the impeller will be fractured or otherwise damaged due to the vibrations and failure to maintain operating clearances. The bearing may be considered to operate on a film of molten metal, and poor concentricity yields reduced clearances which may cause the film to break down or not form so as to give rise to refractory material wear of increased rate.
The pumps and methods are characterized by unique fluid flow properties tending to provide improved pump performance.
In a first aspect of the invention, the fluid flow properties are enhanced by improved trueing or concentricity of the impeller within the base to reduce vibrations and fluid flow irregularities during pumping. More particularly, the impeller is secured to the shaft prior to finish forming or trueing the bearing ring receiving groove. The bearing ring is then secured within the bearing groove. By assuring the concentricity of the bearing within its mounting groove, vibrations due to non-balanced differences in the mass of the materials forming the bearing and impeller are reduced. Thereafter, the peripheral surface of the bearing ring is trued by mechanically shaping it and its concentricity about the shaft axis is assured with machining accuracy. The concentricity of the rotational movement of the bearing ring is thereby further improved and vibration during operation suppressed.
In another aspect of the present invention, an intake feed plate is positioned adjacent the pump inlet to screen or prohibit entry of solid debris into the pump. The feed plate is mounted for rotation, and it also provides a first stage impeller and pumping action that feeds the pump.
The use of volute pumping chamber configurations may be facilitated in accordance with a further aspect of the invention. Heretofore, the volute shape was typically achieved by fitting a circular chamber with a chord or crescent shape insert piece that results in a desired volute shape of increasing radius adjacent the discharge or outlet. More recently, computer numerically controlled machining centers enable direct forming of such designs. The rotational axis of the pump is aligned with the center of the circular chamber, and the volute pumping advantages are provided by altering the shape of the chamber through the use of the insert piece. This prior practice requires additional work in forming the enlarged preliminary circular shape and the subsequent fitting of the insert piece. In contrast with such techniques, the present invention contemplates the provision of a circular pumping chamber and an impeller mounted with its axis of rotation off-set from that of the chamber. In this manner, the volute shape is imparted to the space between the periphery of the impeller and the adjacent surface of the circular pumping chamber.
In yet a further aspect of the present invention, an improved impeller configuration includes peripheral pumping chambers that each have an axial intake through a radial intake opening at the top of the impeller and a radial discharge through an axial outlet opening extending along the outer peripheral side of the impeller. The chambers are disposed at an inclined angle relative to the direction of rotation to impose axial intake vector forces on the molten metal that operate to expedite metal flow into the pumping chamber. Thereafter, the centrifugal forces impose radial forces on the rotating metal causing ejection thereof from the pumping chamber. This impeller configuration thereby imposes two stage pumping and has provided increased pumping effectiveness in that relatively high flows and pressures are achieved.